Field
The present disclosure relates to a micro-traction drive unit as well as a hydrostatic transmission (HST) and a hydraulic device including a micro-traction drive mechanism.
Related Art
For example, JP S58-046252A and JP 3659925B (which will be referred to as Patent Documents 1 and 2, respectively, below) propose micro-traction drive devices comprising a bearing member, such as an angular contact ball bearing or a conical roller bearing, that has an inner ring, a plurality of rolling elements, and an outer ring and is capable of bearing a radial load and a unidirectional axial load, and a retainer that has a retaining part retaining the rolling elements such that the rolling elements are rotatable around their own axes and rotates around the axis of the inner ring in accordance with the orbital motion of the rolling elements around the inner ring.
Such a micro-traction drive device can be utilized as a decelerator when the inner ring is used as an input member and the retainer is used as an output member, and can be utilized as an accelerator when the retainer is used as an input member and the inner ring is used as an output member.
Meanwhile, in order to cause the micro-traction drive device to properly operate as a decelerator or an accelerator, it is necessary to apply a suitable normal force to the bearing member.
The normal force can be obtained, for example, by:
(1) setting the outer diameter of the outer ring and the inner diameter of the inner circumferential surface of a case accommodating the outer ring at such sizes that the outer ring is provided in a state of being compressed in the radially inward direction,
(2) setting the inner diameter of the inner ring and the outer diameter of a power transmission shaft around which the inner ring is disposed in a relatively non-rotatable manner at such sizes that the inner ring is supported in a state of being expanded in the radially outward direction, and/or
(3) configuring one of the outer ring and the inner ring to be movable in a first axial direction and the other to be immovable in the first axial direction and applying an axial load to one of the outer ring and the inner ring by a coil spring (see Patent Documents 1 and 2).
However, with configurations (1) and (2), the magnitude of the normal force applied is determined by the amount of compression of the outer ring and the amount of expansion of the inner ring resulting from the dimensional tolerance, and it is thus difficult to adjust the normal force.
Moreover, in configurations (1) and (2), the outer ring and the inner ring are deformed in the radial direction, and thus the service life may be shortened.
On the other hand, configuration (3) does not suffer the problems encountered with configurations (1) and (2), but the biasing force of the coil spring has to be altered to adjust the normal force, and it cannot be said that the adjustment for obtaining a suitable normal force is easy.
Moreover, since an axial load is applied by the coil spring to the end face of the outer ring (or the inner ring) facing in the axial direction, configuration (3) may be problematic in that uniformly applying an axial load to the entirety of the end face of the outer ring (or the inner ring) is difficult.